FIG. 1A illustrates a typical AC/DC switching boost converter circuit 100 arranged between a power supply and a load. As shown in FIG. 1A, the circuit includes rectifier circuit formed by a diode bridge (with diodes D1 to D4), a boost converter circuit 102 arranged between the diode bridge and a load, an input capacitor Cin connected across the input of the boost converter circuit 102, an output capacitor Co connected across the output of the boost converter circuit 102. In this example, the boost converter circuit 102 is formed by an inductor L, a switching device SW1, and a Zener diode D5. The diode bridge is arranged to rectify an AC input voltage and an AC input current, to provide a rectified input current to the boost converter circuit 102.
FIG. 1B shows waveforms of the supply AC voltage vac, the supply AC current iac, the rectified converter input voltage vin, and the rectified input current iin in the circuit of FIG. 1A. As shown in FIG. 1B, when the capacitor Cin in present, the rectified input current iin is not varying linearly with (e.g., substantially in phase with such that both increase or both decrease at the same time) the rectified converter input voltage vin. As a result, the power factor is degraded and harmonic currents (which increase the total harmonic distortion and generate substantial electromagnetic interference) are created.
Ideally, the rectified input current iin should vary linearly with the rectified converter input voltage vin for achieving unity power factor, zero total harmonic distortion, and zero electromagnetic interference generation.
In this regard, one solution to improve the circuit in FIG. 1A is to remove the capacitor Cin at the input of the switching boost converter circuit and apply continuous mode switching control to the switching boost converter circuit. By doing so, the input current iin can be controlled to have a profile with a near ideal current envelope and with high frequency current component(s) resulting from operation of the switching boost converter circuit (the near ideal current envelope and the high frequency current component(s) are superimposed).
FIG. 2 illustrates waveforms of the supply AC voltage vac, the rectified converter input voltage vin, and the rectified input current iin in such a modified circuit (capacitor Cin removed and continuous mode switching control applied). It can be seen that the power factor has been improved, and the total harmonic distortion and electromagnetic interference have been reduced.
Despite the improvements achieved with such modification, total harmonic distortion and electromagnetic interference for some applications, especially for switching converters with high output power, may still be quite high. Thus, there remains a need to further suppress electromagnetic interference generation and total harmonic distortion in switching converters.